As part of the series on sustainability Newsnight asked Sainsbury's boss Justin King for his views as head of one of the country's leading supermarkets. Often vilified by environmentalists for excessive packaging and food miles, what can supermarkets do to help us work toward a more sustainable future?

We're all concerned about the environment, and the case for change has been convincingly made. But it's important that the change is evidence based. I'm a great believer in examining the facts, and in looking before we leap.

Our customers tell us they want an open and honest debate, so it's vital that business, the media, experts in the field, and government, all come together to educate and inform and lead the right kind of change.

So when our government jumps on the bandwagon of an apparently simple solution, forgetting the issues to be addressed, ignoring the evidence, and indeed an agreement reached with industry - as has happened recently in the debate on plastic bags - I think we all have cause to be concerned.

Bags are in truth a very small part of the plastics and packaging story. Or indeed the wider environmental debate. Yet they've clearly become an icon. Only last week the Marine Conservation Society published a survey of litter on our beaches. Those that read about this would have supposed it was a story of plastic bags; yet they accounted for just 2.2% of the litter collected.

But that's not to say plastic bags aren't important - consumers clearly want to make a difference. But my experience tells me that customers don't want to be forced to make a change, especially where they can't see a clear reason why. They want to be engaged positively with an issue, so that they can really make a lasting difference.

So let's step back - what's the real issue we're addressing here? Well I'd suggest it's the environmental impact in total, rather than the number of bags per se. Bags consume a valuable raw material and they have an environmental impact in their manufacture, their transport, use and disposal. Yet increasingly the debate has become focused on only one dimension of that - the disposal of bags.

Of course not all plastic bags are the same. They may be made from virgin or recycled material, they may be degradable or recyclable. A year ago the industry came together with government through DEFRA and the government's waste reduction body WRAP to debate these issues.

Environmental issues are complex and the solutions are rarely straightforward

Justin King

We looked at evidence from around the world where bans and taxes had been implemented and we concluded the best approach was to focus on reducing the environmental impact. An unprecedented agreement was reached to reduce the environmental impact of plastic bags by 25% by the end of 2008.

In March WRAP reported that a 14% reduction had already been achieved, yet within days the government announced a complete change of tack, and that if more was not done to reduce the number of bags, then a tax would be introduced.

So why not a tax or levy? Well firstly there's little evidence they work, and the alternatives that consumers use are often worse for the environment. And secondly it would be hugely complex: would it include bags made of recycled or degradable material. Indeed would it include paper bags, where the environmental case is often considered to be worse?

So environmental issues are complex and the solutions are rarely straightforward. Take the cucumber - we're often asked why it's packaged. Well a packaged cucumber will stay fresh three times longer - so avoiding waste.

The Sainsbury's boss claims there isn't enough recycling in the UK

So which is worse: a small amount of packaging or a large amount of food waste. And let's move to the vexed issue of air miles, where it's often argued that almost anything flown must be worse for the environment. Yet these Kenyan roses for eight months of the year will have a lower carbon footprint than those grown under glass in Europe.

So let's return to bags. What more could we do? Well I believe the industry focus on reducing, reusing, recycling is the right one. But it's on recycling that we need more help.

Firstly there's a limited ability to recycle in the UK - most plastic is shipped to Germany or the Far East for recycling. And secondly, there is no consistent approach up and down the country on what's able to be recycled. Most councils will recycle plastic bottles for you, but many will recycle little else.

So, rather than chasing headlines, there's two areas that the government could really make a difference: by focusing on ensuring there's a consistent approach to the types of plastic that can be recycled throughout the country; and by ensuring that that recycling takes place here in the UK.

But that will take time, so we as retailers have to work harder at informing our customers, and giving them the opportunity to make real and lasting changes that can really make a difference.

Justin King is in debate with Environment Minister Joan Ruddock on Newsnight on 16 April on BBC Two and on the Newsnight website 2230BST/2130GMT.